Glaucoma involves at least one characteristic change in the optic nerve heads or visual field, and is characterized by both functional and structural abnormalities of eyes. Usually, the optic nerve injury can be improved or prevented from progressing by sufficiently decreasing the ocular pressure. However, glaucoma can lead to blindness if not appropriately treated. Blindness due to glaucoma is the second most common cause of acquired blindness in Japan.
Glaucoma can be classified into primary glaucoma, secondary glaucoma, and developmental glaucoma. Primary glaucoma can be further categorized into primary open-angle glaucoma (broadly defined), primary angle-closure glaucoma, and mixed glaucoma. Broadly-defined primary open-angle glaucoma includes primary open-angle glaucoma (narrowly defined) and normal tension glaucoma. Normal tension glaucoma is a disease in which the optic nerve is damaged, although the intraocular pressure is within the normal range (21 mmHg or less, 15.5 mmHg on average). Approximately 5.8% of people who are 40 years old or older are said to be affected by glaucoma. Since, according to the statistics in 2000, the 40 and older population in Japan is approximately 65 million, the number of people affected by glaucoma who are 40 years old or older is estimated to be over 3.7 million.
“Intraocular pressure” is an important risk factor in the occurrence and progression of optic nerve injury associated with glaucoma. Conventionally, decreasing intraocular pressure has been recognized as the only reliable therapeutic method. Therapeutic methods for decreasing the intraocular pressure generally involve eye drops (β-blockers, prostaglandin-related agents, carbonic anhydrase inhibitors, and such), oral or injection agents (carbonic anhydrase inhibitors, or hypertonic agents), and surgery (laser surgery, or invasive surgery).
However, factors other than “intraocular pressure”, such as impaired microcirculation and fragility of the optic nerves, have also been suggested to be involved in glaucoma, and the limit of intraocular pressure-lowering therapies has been pointed out. Therefore, there is a need to develop therapeutic methods for glaucoma apart from intraocular pressure-lowering therapies. One such method that has attracted attention involves inhibiting retinal ganglion cell death (apoptosis), the final pathology of glaucoma; namely, retinal ganglion cell protection therapy.
Meanwhile, neurotrophic factors promote growth and differentiation of undifferentiated neuroblasts, as well as the survival and maintenance of the function of mature neurons. Pigment epithelium-derived factor (PEDF) is one of the neurotrophic factors. To date, two biological activities for PEDF have been reported: neurodifferentiation/neuroprotection activity and antiangiogenic activity. PEDF was originally purified in 1989 from the culture supernatant of human embryonic retinal pigment epithelial cells as a factor that promotes neurodifferentiation of human Y-79 retinoblastoma cells (Non-Patent Document 1). It has since been reported to have effects of inducing differentiation and suppressing injury-induced neuronal apoptosis of various nerve cells, in both in vitro and in vivo systems. The underlying mechanisms have been examined using cultured immature cerebellar granule cells. It has been reported that activation of the transcription factor, NFκB, is involved in these mechanisms and that the expression of the anti-apoptotic genes, Bcl-2 and Bcl-x, and the neurotrophic factors, NGF and BDNF, is also induced (Non-Patent Document 2). Meanwhile, in a microarray study for cultured immature cerebellar granule cells, it has been reported that PEDF addition induces the expression of various neurotrophic factors (NGF, neurotrophin-3, and GDNF), though neurotrophic factors induced in the analysis using neutralizing antibodies do not influence the neuroprotective effect of PEDF (Non-Patent Document 3), which suggests that the protective effect is a direct action of PEDF. Furthermore, in 1999, it was reported that PEDF suppressed FGF-2-induced migration of vascular endothelial cells in a concentration-dependent manner in in vitro systems. This effect was higher than angiostatin or endostatin. In addition, PEDF was also shown to significantly suppress FGF2-induced corneal neovascularization in vivo (Non-Patent Document 4). Thereafter, a number of reports have been made on the phenomena of suppressing various angiogenesis models and tumor angiogenesis. Their mechanisms have not been elucidated in detail, but the following possibilities are contemplated: (1) since PEDF induces the expression of FasL in vascular endothelial cells, and Fas is highly expressed in vascular endothelial cells that are in the process of neogenesis, Fas/FasL-mediated apoptosis of endothelial cells may suppress angiogenesis (Non-Patent Document 5); (2) extracellular phosphorylation may be involved (Non-Patent Document 6); and (3) binding with extracellular substrates may be involved.
Based on the apoptosis-suppressing effects described above, methods for protecting ganglion cells using neurotrophic factors have been examined. To date, two studies of PEDF gene therapy using retinal ischemia reperfusion models have been reported. In these studies, the cell injury-suppressing effect of PEDF was examined using “retinal ischemia reperfusion model” rats, whose ganglion cells are damaged and have undergone apoptotic death as in glaucoma. In the above-mentioned studies, a PEDF protein (Non-Patent Document 7) or an adenoviral vector carrying PEDF (Non-Patent Document 8) was administered to the vitreous body of the animals, and retinal ganglion cell injury due to ischemia reperfusion was suppressed histologically.
However, neurotrophic factors have a large molecular weight. It is difficult to continuously deliver large molecular weight proteins to the retina using the current drug delivery systems. Furthermore, since genes introduced by adenoviral vectors exist as episomes in nuclei and are thus not incorporated into chromosomal DNA, transgenes that do not have autonomous replication ability are diluted as the cells grow, and expression of the transgenes becomes transient. Considering that glaucoma is a chronic disease, administration methods that are expected to provide only transient effects cannot be considered as suitable therapeutic methods for glaucoma. On the other hand, retroviral vectors may generally enable long-term expression of genes by being stably incorporated into the chromosomes of dividing cells; however, there is so far no known study of glaucoma therapy that uses retroviral vectors into which PEDF has been inserted.    [Patent Document 1] International Application No. PCT/JP2002/005225; WO2002/101057    [Patent Document 2] International Application No. PCT/JP00/03955; WO00/078987    [Non-Patent Document 1] Tombran-Tink J, Chader G G; Johnson L V. PEDF: a pigment epithelium-derived factor with potent neuronal differentiative activity. Exp Eye Res. 1991 September; 53(3):411-4.    [Non-Patent Document 2] Yabe T, Wilson D, Schwartz J P. NFkappaB activation is required for the neuroprotective effects of pigment epithelium-derived factor (PEDF) on cerebellar granule neurons. J Biol. Chem. 2001 November 16; 276(46):43313-9.    [Non-Patent Document 3] Yabe T, Herbert J T, Takanohashi A, Schwartz J P. Treatment of cerebellar granule cell neurons with the neurotrophic factor pigment epithelium-derived factor in vitro enhances expression of other neurotrophic factors as well as cytokines and chemokines. J Neurosci Res. 2004 September 1; 77(5):642-52.    [Non-Patent Document 4] Dawson D W, Volpert O V, Gillis P, Crawford S E, Xu H, Benedict W, Bouck N P. Pigment epithelium-derived factor: a potent inhibitor of angiogenesis. Science. 1999 July 9; 285(5425):245-8.    [Non-Patent Document 5] Volpert O V, Zaichuk T, Zhou W, Reiher F, Ferguson T A, Stuart P M, Amin M, Bouck N P. Inducer-stimulated Fas targets activated endothelium for destruction by anti-angiogenic thrombospondin-1 and pigment epithelium-derived factor. Nat. Med. 2002 April; 8(4):349-57.    [Non-Patent Document 6] Maik-Rachline C; Shaltiel S, Seger R. Extracellular phosphorylation converts pigment epithelium-derived factor from a neurotrophic to an antiangiogenic factor. Blood. 2005 January 15; 105(2):670-8. Epub 2004 Sep. 16.    [Non-Patent Document 7] Ogata N, Wang L, Jo N, Tombran-Tink J, Takahashi K, Mrazek D, Matsumura M. Pigment epithelium derived factor as a neuroprotective agent against ischemic retinal injury. Curr Eye Res. 2001 April; 22(4):245-52.    [Non-Patent Document 8] Takita H, Yoneya S, Gehlbach P L, Duh E J, Wei L L, Mori K. Retinal neuroprotection against ischemic injury mediated by intraocular gene transfer of pigment epithelium-derived factor. Invest Opthalmol Vis Sci. 2003 October; 44(10):4497-504.    [Non-Patent Document 9] Miyazaki M, Ikeda Y, Yonemitsu Y, Goto Y, Sakamoto T, Tabata T, Ueda Y, Hasegawa M, Tobimatsu S, Ishibashi T, Sueishi K. Simian lentiviral vector-mediated retinal gene transfer of pigment epithelium-derived factor protects retinal degeneration and electrical defect in Royal College of Surgeons rats. Gene Ther. 2003 August; 10(17):1503-11.